Silicon carbide is the fifth hardest known material behind diamond. The high hardness of SiC combined with its relatively low weight (theoretical density: 3.21 g/cm3) and cost have made SiC one of the preferred materials for the strike face of personal and vehicular armor systems, to stop; for example, armor piercing bullets.
The high temperature stability and creep resistance, chemical stability, and abrasion resistance of SiC have also made silicon carbide a prominent material in refractory, wear, seal, and chemical processing applications.
Prochazka et al. (S. Prochazka and R. M. Scanlan, “Effect of Boron and Carbon on Sintering of Silicon Carbide,” J. Am. Ceram. Soc., 58 [1-2] 72 (1975)) were the first to report a process for forming dense (about 96.4% relative density) SiC microstructures through solid-state pressureless sintering. In that process, phenolic resin (0.25-0.5 wt %), a carbon source after pyrolysis, and B4C (0.36 wt %) were added to fine (0.13 μm) β-SiC powders. The carbon additive was believed to be responsible for the removal of SiO2 coatings on SiC particles (2C(s)+SiO2(1)=SiC(s)+CO2(g)), and B4C was believed to enhance solid state diffusion by near-grain boundary vacancy formation (via limited solubility of boron in the SiC structure). As opposed to liquid phase sintered SiC (facilitated, for example, by Y2O3 and Al2O3 additions), solid state sintered SiC has a higher hardness, better chemical durability and high temperature creep resistance.
PCT/US2011/106768 discloses a process for the fabrication of boron carbide parts with no porosity with a median grain size approaching the starting median particle size (d50=0.8 μm), and higher Vickers hardness values than previously obtainable by pressureless sintering techniques. That process is based on the use of two water-soluble chemical additives yielding sintering aids/grain growth inhibitors after thermal decomposition, careful selection of heat-treatment schedules, and utilization of a sub-micron B4C powder of narrow particle size distribution. In that process, carbon additions in the form of water-soluble phenolic resin, and titanium additions in the form of a water-soluble source (C6H10O8Ti.2(NH4)) facilitate intimate coating of the boron carbide particles in the boron carbide powder with a titanium source and a carbon source. The sources of titanium and carbon are then thermally decomposed (preferably through pyrolysis) after the coated boron carbide particles are formed into a green body. The green body is subjected to pressureless sintering after it is subjected to a heat treatment step to pyrolyze the titanium and the carbon sources.